The reaction of polymeric [{Mo(NO)₂Cl₂}_n] with ligands L or L₂[L = MeCN, CH₂CHCN, PhCN, PPh₃, or pyridine; L₂= 2,2′-bipyridine (bipy) or 1,2-bis(diphenylphosphino)ethane] affords neutral monomenc [Mo(NO)₂L₂Cl₂] in high yield. The structure of [Mo(NO)₂(bipy)Cl₂] has been determined by X-ray diffraction. The compound crystallises in the monoclinic space group P2₁/c with a= 8.1 42(3), b= 16.383(6), c= 11.948(3)Å, β= 121.43(4)°, and Z= 4. The structure was solved by conventional Patterson and Fourier methods to a conventional R= 0.045, R′= 0.050 for 1 687 independent reflections having |F_o|² > 3σ|F_o|². The crystal structure consists of well separated, discrete molecules. The co-ordination geometry around the molybdenum is described as distorted octahedral with cis-dinitrosyl-trans-dichloro-stereochemistry. The two NO groups are ordered and nearly linear [Mo–N–O = 175.9(10) and 177.4(13)°]. The characterization of all of the dinitrosylmolybdenum complexes by vibrational, ¹H and ³¹P n.m.r., and u.v.–visible spectroscopy is described. From a comparison of the spectroscopic studies with the X-ray structures of [Mo(NO)₂(bipy)Cl₂] and the other known compound [Mo(NO)₂(PPh₃)₂Cl₂], two kinds of stereochemistry are formed cis-dinitrosyl-trans-dichloro and cis-dinitrosyl-cis-dichloro.